Finally found the working code. Install userChromeJS extension, create chrome/userChrome.js file with the next code:

document.addEventListener("DOMContentLoaded", () => {
  ZoomManager.setZoomForBrowser(getBrowser(), 1.2);
});

Works for the email preview and message composition window.
Kudos to Reddit guys, their discussion gave me an idea, how my command should be invoked properly.